When long-distance transmission is performed using wavelength division multiplexing (WDM), a dispersion compensating fiber is generally arranged at each repeater station and a receiver station to perform correction for cancelling an amount of chromatic dispersion received during the preceding transmission so that a light signal may be received successfully and, in addition, a tunable dispersion compensator (TDC) is generally arranged in front of a receiver module to perform optimum dispersion compensation on each channel.
However, since individual wavelength-division multiplexed channels receive different amounts of dispersion from a transmission path, a difference or variance is caused in an amount of residual dispersion (amount of accumulated dispersion) between the channels even if dispersion compensation is performed at repeater stations. This difference or variance increases in proportion to the transmission distance.
FIG. 1 is a diagram illustrating dispersion compensation and residual dispersion in WDM transmission. Different amounts of dispersion are accumulated for individual wavelengths in each transmission path. The longer the wavelength of a channel, the more easily the channel is affected by dispersion and the greater the amount of residual dispersion (arrow A). The amount of chromatic dispersion is decreased at each repeater station by using a dispersion compensating fiber (arrow B). However, since the amount of accumulated dispersion and the amount of compensation differ depending on the wavelength, it is difficult to return the amounts of residual dispersion of all channels to zero (arrow C).
Since different amounts of dispersion compensation are desirably applied to individual signal wavelengths at a receiver station, a tunable dispersion compensator (TDC) is arranged on the reception side. In order to support a range between the maximum value and the minimum value of the amounts of compensation of all channels by using one TDC, a chromatic dispersion compensation range of the TDC is desirably made wide (arrow D). However, an amount of tunable chromatic dispersion and the bandwidth of an effective dispersion compensation band of a TDC have a tradeoff relationship as illustrated in FIG. 2.
As illustrated in FIG. 3A, the effective dispersion compensation band indicates a band within which an amount of dispersion has linearity, and is generally defined as a wavelength range within which the ripple or slope of group delay is smaller than or equal to a given value. On the other hand, as illustrated in FIG. 3B, a transmission band is defined as a wavelength range by points which are lower than the minimum loss value (peak value) by, for example, 3 dB.
Referring to FIG. 2, when the effective dispersion compensation band is widened, the tunable range of the amount of dispersion compensation narrows (arrow D). When the tunable range is widened, the effective dispersion compensation band narrows (arrow E). Accordingly, it is difficult to achieve both a wide effective wavelength band and a large amount of tunable dispersion compensation by using one device.
As illustrated by arrow F in FIG. 4A, the slope in the range of the effective dispersion compensation band where the linearity is maintained corresponds to an amount of chromatic dispersion. In this linearity-maintained range, a uniform amount of chromatic dispersion compensation is applicable. The same amount of dispersion compensation is not applicable to all signal spectrum components unless both the effective dispersion compensation band and the transmission band are wider than spectrum width of signal light. If the effective dispersion compensation band is not wide enough, transmission penalties are caused by deterioration of the signal waveform as illustrated by arrow G in FIG. 4B. Accordingly, dispersion compensation is reliably performed based on narrower one of the effective dispersion compensation band and the transmission band.
To perform dispersion compensation on an approximately 40-GHz signal, a band of approximately 50 GHz is desirably obtained. However, it is difficult to support an amount of dispersion compensation of ±1000 ps/nm after obtaining this band. For this reason, a general TDC has an amount of tunable dispersion compensation of approximately ±700 ps/nm.
A configuration has been proposed in which two TDCs are coupled in series in order to obtain a desired dispersion compensation band (e.g., Japanese Laid-open Patent Publication No. 2010-288200). In this configuration, a second TDC is cascade coupled in order to compensate for the shortage of the dispersion compensation band of a first TDC. The total amount of tunable dispersion compensation may be increased by coupling multiple TDCs in series. However, the band may undesirably narrow depending on the way of control, and as a result the transmission quality may decrease.
In addition, a configuration is known in which optical dispersion compensation elements are coupled in series in upstream of a stage where wavelength demultiplexing is performed on the reception side, in order to compensate for the third-order dispersion for entire transmission signal light (e.g., Japanese Laid-open Patent Publication No. 2001-320328).
Furthermore, a configuration has bee proposed which realizes a dispersion compensation device having a wide band and a few ripple by coupling multiple TDCs in each of which an etalon and a mirror are arranged at a given angle (e.g., Japanese Laid-open Patent Publication No. 2006-053519).
FIGS. 5A and 5B are diagrams for describing characteristic differences resulting from different TDC combinations. For example, when an amount of dispersion compensation of ±800 ps/nm is applied in total by coupling two TDCs each having a tunable dispersion wavelength characteristic of ±700 ps/nm, following example combinations are possible. Although there are an infinite number of combinations, two examples will be illustrated herein for convenience.+700 ps/nm(TDC 1)+100 ps/nm(TDC 2)=+800 ps/nm (FIG. 5A)  Method 1+400 ps/nm(TDC 1)+400 ps/nm(TDC 2)=+800 ps/nm (FIG. 5B)  Method 2
In the method 1, a large dispersion value (+700 ps/nm) is set for one of the TDCs as illustrated in FIG. 5A. The effective dispersion compensation band of the TDC having the large dispersion value is narrow. When the two TDCs are coupled in series, the characteristic of the entire device is affected by the characteristic of the TDC having the narrower band. As a result, the effective bandwidth of the entire device narrows.
On the other hand, when two TDCs each having a dispersion value of +400 ps/nm are coupled, both of the TDCs have substantially even effective dispersion compensation bands as illustrated in FIG. 5B. Each TDC has the band that is narrower than the effective dispersion compensation band corresponding to +100 ps/nm but that is wider than the effective dispersion compensation band corresponding to +700 ps/nm. When the two TDCs are coupled, a band that is wider than the band obtained by the combination illustrated in FIG. 5A may be obtained as a whole.